The invention relates to an electrolysis process and electrolytes therefor for producing an alkali metal.
Alkali metals are highly reactive elements and are not found in elemental form in nature. Typical reducing agents, such as hydrogen, are not strong enough to reduce the alkali metals from their compounds to the metallic state. Electrolytic reduction is necessary and was used historically in the classic experiments leading to the discovery of the alkali metals in elemental form in 1807 by Sir Humphrey Davy. Electrolytic reduction is used for industrial production of the alkali metals. The currently used process, on a worldwide basis, is the so-called xe2x80x9cDownsxe2x80x9d Process, which was introduced in the early part of the 20th century for the production of sodium and lithium from their chlorides.
The Downs Process uses a molten salt electrolyte consisting of a mixture of NaCl, CaCl2 and BaCl2 in order to reduce the melting temperature of the electrolyte to slightly below 600xc2x0 C. This makes the process more practical compared to using pure NaCl which has a much higher melting point of about 800xc2x0 C. Nevertheless, operating an electrolytic process at such temperature is difficult and presents numerous operating constraints. Because of the high operating temperature of the Downs Process, the cell design uses concentrically cylindrical cathodes, wire mesh diaphragms, and anodes rather than the much more space efficient stacked multiple flat electrode and diaphragm element configuration that is normally used in electrochemical engineering practice. Furthermore, the high operating temperature would make a flat wire-mesh steel diaphragm so soft that it would be mechanically unstable and flap back and forth between anode and cathode causing partial shorting/arcing and thereby causing holes to be burned in the diaphragm. Holes in the diaphragm would allow back mixing of sodium produced at the cathode and chlorine produced at the anode, thereby causing low current efficiency of the cell. On the other hand, the concentric cylindrical configuration of the steel diaphragm between the electrodes avoids this difficulty because a wire-mesh cylinder is mechanically much stiffer and mechanically more stable than a flat wire-mesh screen of the same kind.
The above-described concentric cylindrical cell design of the Downs Process, necessitated by the high operating temperature of about 600xc2x0 C., also means that the Downs cell has very poor space efficiency. This translates directly into high capital and operating cost per unit production.
The high operating temperature of the Downs cell in combination with the fact that the molten mixed salt electrolyte has a freezing temperature only about 20xc2x0 C. below the cell operating temperature makes smooth operation of the cells difficult. Cell xe2x80x98freeze-upsxe2x80x99 and other xe2x80x9cupsetsxe2x80x9d are frequent and result in unusually high operating labor requirements for an industrial electrolytic process. This in turn is also the reason why the Downs Process is not amenable to automation. Lithium is currently produced by a modification of the Downs process.
In recent years fundamental physico-chemical studies have been carried out on battery applications using electrolytes based on non-aqueous solvents for alkali metal chlorides that do not crystallize at ambient temperature. See J. Electrochem. Soc. Vol. 143 No. 11, pages 3548-3554, November 1996; and U.S. Pat. No. 5,855,809, disclosures of which are incorporated by reference.
There is an increasing need to develop an electrolytic process that can be used to produce an alkali metal more economically. There is also a need to develop a process that can improve operability such as, for example, making automation possible.
An electrolysis process that can be used for producing an alkali metal from an alkali metal halide is provided, which comprises electrolyzing an electrolyte composition comprising, or produced by combining, at least one alkali metal halide and a co-electrolyte in which the co-electrolyte comprises or is produced by combining (a) at least one halide of Group IIIA, Group IB, or Group VIII and (b) a halide-donating compound.